1. Field of the Invention
The present invention relates to a light-emitting device and a method of manufacturing the light-emitting device. The present invention also relates to an evaporation donor substrate used for deposition of a material which can be deposited onto a substrate.
2. Description of the Related Art
Organic compounds can take various structures compared with inorganic compounds, and it is possible to synthesize materials having various functions by appropriate molecular design. Owing to these advantages, photo electronics and electronics which utilize functional organic materials have been attracting attention in recent years.
A solar cell, a light-emitting element, an organic transistor, and the like are given as examples of electronics devices using an organic compound as a functional organic material. These devices take advantage of electrical properties and optical properties of the organic compound. Among them, in particular, a light-emitting element has been making remarkable progress.
It is said that light-emitting elements have a mechanism of light emission as follows: by applying voltage between a pair of electrodes with an electroluminescence (hereinafter also referred to as EL) layer interposed therebetween, electrons injected from a cathode and holes injected from an anode recombine with each other in an emission center of the EL layer to form molecular excitons, and energy is released when the molecular excitons relax to the ground state; accordingly light is emitted. As excited states, a singlet excited state and a triplet excited state are known, and light emission is considered to be possible through either of these excited states.
An EL layer included in a light-emitting element includes at least a light-emitting layer. In addition, the EL layer can have a stacked-layer structure including a hole-injecting layer, a hole-transporting layer, an electron-transporting layer, an electron-injecting layer, and/or the like, in addition to the light-emitting layer.
In addition, EL materials for forming EL layers are broadly classified into a low molecular (monomer) material and a high molecular (polymer) material. In general, a low molecular material is often deposited using an evaporation apparatus and a high molecular material using an inkjet method or the like. A conventional evaporation apparatus, in which a substrate is mounted in a substrate holder, has a crucible (or an evaporation boat) containing an EL material, that is, an evaporation material, a heater for heating the EL material in the crucible, and a shutter for preventing the subliming EL material from being scattered. Then, the EL material heated by the heater is sublimed to be deposited onto the substrate. At this time, in order to achieve uniform deposition, a deposition target substrate needs to be rotated, and further the distance between the substrate and the crucible needs to be about 1 m even for a 300 mm by 360 mm substrate.
In the case of considering manufacturing a full-color flat panel display using emission colors of red, green, and blue by the above method, a metal mask is provided between the substrate and an evaporation source so as to be in contact with the substrate, so that selective coloring can be achieved using this mask. However, this method does not provide very high-precision deposition and thus requires that the distance between pixels be designed to be large and that the width of a partition wall (a bank) formed of an insulator between pixels be large. Therefore, application of the method to a high-definition display device is difficult.
In addition, demands for more definition, higher aperture ratio, and higher reliability of a full-color flat panel display using emission colors of red, green, and blue have been increasing. Such demands are major issues in advancing miniaturization of each display pixel pitch, which is associated with improvement in definition (an increase in the number of pixels) and a reduction in size of a light-emitting device. At the same time, demands for more productivity and lower cost have also been increasing.
Thus, a method of forming an EL layer of a light-emitting element by laser thermal transfer has been proposed (see Reference 1: Japanese Published Patent Application No. 2006-309995). Reference 1 discloses a transfer substrate which has a photothermal conversion layer including a low-reflective layer and a high-reflective layer and also a transfer layer over a supporting substrate. Irradiation of such a transfer substrate with laser light (also refereed to as laser beam) allows the transfer layer over the low-reflective layer to be transferred to an element-forming substrate.